Most of the times squeky cranksets come from bottom bracket issues. Pull the bottom bracket clean the shell threads and the bracket threads, use teflon tape on the bracket threads, a little grease and reinstall. This usually does the trick. However, sometimes the squeks are not coming from where you think. Could be seatpost, pedals, seat, etc..

This is probably a stupid question but have you checked your pedals? Try spinning the crankset using one pedal at a time. If the noise never goes away then it is probably the bottom bracket. If it only makes the noise with one pedal then try a little oil on the pedal spindle. Try this before doing what bbarend suggested. It only takes a second and could avoid a headache.